The estimated number of HCV carriers is about 170 million worldwide (about 3%) and about 1.5 million in Japan. Even in the combination therapy of using interferon (hereafter referred to as IFN) and ribavirin (Virazole), available as a first option for treatment, its effectiveness is 40% for all types of HCV. Furthermore, its effectiveness is only 15 to 20% for IFN-resistant virus (genotype 1b), particularly abundantly found in Japan. On the other hand, the combination therapy has side effects frequently. It is thus difficult to get rid of the virus completely by using currently available treatment methods. In the case when chronic hepatitis cannot be cured completely, the hepatitis will securely develop into cirrhosis hepatitis (30%) or hepatocellular carcinoma (25%). In Europe and the United States, hepatitis C has been a major indication for liver transplant. However, the redevelopment of HCV occurs frequently even in transplanted livers. For these reasons, the needs for new agents being improved in both effectiveness and safety, having higher antiviral effects and capable of inhibiting hepatitis C are very strong in society.
HCV is a virus having plus-strand RNA as a gene and is classified into Flaviviridae in accordance with the analysis of the base sequence of the gene. According to Fields Virology fourth edition, D. Knipe et al ed., Philadelphia, Lippincott Williams & Wilkins 2001, 1127-1161, although the existence of HCV was anticipated in 1970s, the discovery of HCV was very difficult. HCV was called non-A non-B hepatitis virus for many years. In 1989, according to Choo Q-L et al., Science 244, 359-362 (1989), part of the gene of this virus was cloned from the serum of an infected laboratory animal, and its cDNA sequence was identified and confirmed, whereby the virus was named “HCV.”